049  
FXUS02 KWBC 050659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST SUN DEC 05 2021  
 
VALID 12Z WED DEC 08 2021 - 12Z SUN DEC 12 2021  
 
...SOME SNOW IS POSSIBLE ON WEDNESDAY FOR PORTIONS OF THE  
NORTHEAST AND NORTHERN MID-ATLANTIC, POTENTIALLY INCLUDING THE  
I-95 CORRIDOR...  
 
...CONSIDERABLE MOUNTAIN SNOWS ARE LIKELY ACROSS THE CASCADES,  
SIERRA NEVADA, INTERMOUNTAIN WEST, AND ROCKIES DURING THE LATTER  
HALF OF NEXT WEEK...  
   
..OVERVIEW
 
 
THE MEDIUM RANGE PERIOD BEGINS WEDNESDAY WITH A BROAD MEAN TROUGH  
ACROSS MUCH OF THE LOWER 48, WITH EMBEDDED SHORTWAVES PUSHING LOW  
PRESSURE/FRONTAL SYSTEMS ACROSS THE COUNTRY, LEADING TO A  
CONTINUING ACTIVE WEATHER PATTERN. EXPECT ONE SYSTEM TO SPREAD  
WIDESPREAD RAIN TO THE SOUTHEAST ON WEDNESDAY WHILE NORTHEASTERN  
PARTS OF THE U.S. MAY SEE SNOW. AS ENERGY DROPS SOUTHWARD THROUGH  
THE WESTERN SIDE OF THE BROAD TROUGH THURSDAY INTO FRIDAY,  
TROUGHING IN THE WEST WILL AMPLIFY AND MOVE INTO THE CENTRAL U.S.  
BY THE WEEKEND. THIS WILL BRING THE POTENTIAL FOR MEANINGFUL  
PRECIPITATION AND HEAVY MOUNTAIN SNOWS IN THE WEST, WITH  
PRECIPITATION SPREADING INTO THE CENTRAL AND EASTERN U.S. BY LATE  
IN THE WEEK ALONG AND AHEAD OF A STRONG COLD FRONT.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
WHILE THE OVERALL LARGE SCALE PATTERN REMAINS AGREEABLE AMONG  
MODEL GUIDANCE, EMBEDDED SHORTWAVE UNCERTAINTIES CONTINUE TO LOWER  
CONFIDENCE IN SOME IMPORTANT DETAILS. ONE NOTABLE EXAMPLE IS WITH  
SHORTWAVE ENERGY LOCATED NEAR THE NORTH-CENTRAL U.S. EARLY  
WEDNESDAY AND ITS DEPTH AND EVOLUTION AS IT TRACKS EASTWARD LATE  
WEDNESDAY INTO THURSDAY. THERE IS A CLEAR SIGNAL FOR THE  
SHORTWAVE'S EXISTENCE BUT SUBTLE LOW-PREDICTABILITY DIFFERENCES  
LEAD TO VERY DIFFERENT SURFACE SOLUTIONS FOR LOW PRESSURE THAT  
TRACKS OFF THE EAST COAST ALONG WITH ASSOCIATED COVERAGE/INTENSITY  
OF PRECIPITATION AND LOCATION OF THE RAIN-SNOW LINE. GFS RUNS  
CONTINUE TO BE AMONG THE SLOWEST AND DEEPEST WITH THE MIDLEVEL  
SHORTWAVE AND THUS THE SURFACE LOW TRACKS FARTHER WEST CLOSER TO  
THE NORTHEAST COAST COMPARED TO CONSENSUS. THIS WAS PARTICULARLY  
TRUE WITH THE 12Z RUN BUT THE 18Z AND 00Z RUNS ARE SOMEWHAT WEST  
AS WELL. THE OVERALL TREND (INCLUDING THE NEW 00Z ECMWF) HAS BEEN  
FOR A MORE SUPPRESSED SOLUTION WITH A LOW TRACK FLATTER AND  
FARTHER OFFSHORE, MEANING LESS PRECIPITATION INDICATED FOR THE  
NORTHEASTERN QUADRANT OF THE CONUS OVERALL, THOUGH ALSO A COLDER  
TREND, SO PRECIPITATION COULD BE MORE LIKELY TO FALL AS SNOW.  
FAVORED AN INTERMEDIATE SOLUTION WITH SOME COMPONENT OF THE 18Z  
GFS BUT COMBINED WITH THE 12Z ECMWF/CMC/UKMET FOR THE EARLY PART  
OF THE MEDIUM RANGE PERIOD. OTHER SOUTHERN STREAM SHORTWAVE  
DIFFERENCES IN THE SOUTHWEST/EASTERN PACIFIC EARLY IN THE FORECAST  
PERIOD FORTUNATELY HAVE LESS SENSIBLE WEATHER IMPACTS.  
 
CONSIDERING THE HIGH LATITUDE/LOW PREDICTABILITY ORIGINS OF  
SHORTWAVE ENERGY DROPPING SOUTHWARD SERVING TO AMPLIFY THE TROUGH  
IN THE WEST, MODEL GUIDANCE IS PRETTY AGREEABLE WITH THE OVERALL  
IDEA OF THE TROUGH DEEPENING ON FRIDAY AND PUSHING EASTWARD  
THROUGH THE WEEKEND. HOWEVER INDIVIDUAL RUNS/ENSEMBLE MEMBERS DO  
STILL VARY CONSIDERABLY FOR EXACTLY HOW ENERGY WILL BE DISTRIBUTED  
WITHIN THE OVERALL TROUGH, AND WHILE THERE IS FAIRLY GOOD  
CONSENSUS FOR THE POSITION OF A COLD FRONT AHEAD OF THE TROUGH,  
POSITIONS OF LOW PRESSURE WAVES ALONG THE FRONT VARY WITHIN A  
CORRIDOR BETWEEN THE GREAT LAKES AND SOUTHERN PLAINS ON SATURDAY  
AND IN THE EASTERN U.S. BY SUNDAY. AS THE PERIOD PROGRESSED,  
TRANSITIONED TO A 60% DETERMINISTIC AND 40% ENSEMBLE MEAN (EC AND  
GEFS) BLEND TO TEMPER INDIVIDUAL MODEL DIFFERENCES IN THE FORECAST.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
GULF MOISTURE IN PLACE BY WEDNESDAY AHEAD OF FRONTAL SYSTEMS WILL  
LEAD TO WIDESPREAD RAINFALL ACROSS THE SOUTHEAST. SHOWERS ARE  
FORECAST TO LINGER THERE THROUGH THE LATTER PART OF THE WORKWEEK  
AS A WAVY FRONT SLOWLY LIFTS NORTHWARD. MOISTURE SPREADING FARTHER  
NORTH INTO COOLER AIR MAY PRODUCE SNOW ACROSS THE NORTHERN  
LATITUDES ON WEDNESDAY, WITH THE HIGHEST PROBABILITIES OF SNOW  
ACROSS PARTS OF THE NORTHEAST. THERE IS STILL CONSIDERABLE  
UNCERTAINTY REGARDING HOW LOW PRESSURE FROM THE EASTERN U.S. INTO  
THE WESTERN ATLANTIC WILL EVOLVE/TRACK, SO CONFIDENCE IS FAIRLY  
LOW FOR EXACTLY HOW MUCH PRECIPITATION MAY FALL OVER THE  
NORTHEAST, AND THE EXTENT OF NOTABLE SNOW.  
 
AS SHORTWAVE ENERGY AND TROUGHING DROP SOUTHWARD THROUGH THE WEST,  
PRECIPITATION WILL SPREAD FROM THE PACIFIC NORTHWEST AND NORTHERN  
ROCKIES WEDNESDAY SOUTHWARD TO CALIFORNIA INTO THE GREAT BASIN TO  
CENTRAL ROCKIES THURSDAY AND FRIDAY. FAVORED TERRAIN IN ARIZONA  
COULD ALSO SEE SOME ENHANCED ACTIVITY. GENERALLY EXPECT HIGHER  
ELEVATION SNOW AND LOWER ELEVATION RAIN, WITH SNOW LEVELS LIKELY  
TO DECLINE LATER IN THE WEEK AS UPPER TROUGHING AMPLIFIES OVER THE  
WEST. PARTS OF SOUTHERN CALIFORNIA MAY SEE THE FIRST RAINFALL IN  
OVER A MONTH. THEN AS THE WESTERN UPPER TROUGH APPROACHES/PUSHES  
INTO THE PLAINS TOWARD THE END OF THE WEEK, RAINFALL MAY BECOME  
MODERATE TO HEAVY OVER PARTS OF THE EAST-CENTRAL U.S. BY NEXT  
SATURDAY WITH THE AID OF STRENGTHENING GULF INFLOW AHEAD OF A WAVY  
FRONT. THE CHARACTER OF ONE OR MORE FRONTAL WAVES WILL DETERMINE  
PRECIPITATION AMOUNTS AND POSSIBLE WINTER WEATHER IMPACTS OVER THE  
PLAINS AND MIDWEST.  
 
TEMPERATURES WEDNESDAY WILL BE ABOUT 10-15F ABOVE AVERAGE FOR THE  
HIGH PLAINS, AND EXPECT TEMPERATURES TO ONLY WARM FROM THERE  
ACROSS THE CENTRAL AND EVENTUALLY EASTERN U.S. AS THE PATTERN  
AMPLIFIES. BY FRIDAY TEMPERATURES IN THE SOUTHERN PLAINS AND THE  
LOWER/MIDDLE MISSISSIPPI VALLEY ARE FORECAST TO BE 15-25F ABOVE  
NORMAL, WITH HIGHS NEARING OR EXCEEDING 80F IN PARTS OF TEXAS,  
WHICH COULD APPROACH RECORDS. MILD TEMPERATURES WITH HIGHS 15-25F  
AND LOWS 15-30F ABOVE NORMAL WILL SPREAD INTO THE EASTERN U.S. FOR  
THE WEEKEND, WHILE CENTRAL U.S. TEMPERATURES SHOULD MODERATE BACK  
TO NORMAL BEHIND THE POTENT COLD FRONT. MEANWHILE, COOLER THAN  
AVERAGE TEMPERATURES BY AROUND 10-15F ARE FORECAST FOR PARTS OF  
THE WEST PARTICULARLY ON FRIDAY AND SATURDAY GIVEN THE AMPLIFYING  
UPPER TROUGH.  
 
TATE/RAUSCH  
 
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDICES ARE AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 

 
 
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